This invention relates to electrical cables, but more particularly, to dual layer jacket cables.
The National Electrical Codexe2x80x94NEC (and CECxe2x80x94Canadian Electric Code for Ontario and B.C.) requires the use of metal conduits for communication cables installed in the return-air plenums of office buildings; an exception to this requirement is granted by NEC and CEC provided that such cables are approved as having low flame spread and smoke producing characteristics. In order to gain this approval, the cables are tested by independent laboratories in accordance to the UL-910/NFPA 262 Standard Test Methods for Fire and Smoke Characteristics of Cables Used in Air Handling Spaces and must pass its requirements.
In addition to the safety requirements mandated by the NEC articles, modern communication cables must meet electrical performance characteristics equivalent or better than required for transmission frequencies of up to at least 100 Mhz, as presently specified by ANSI/TIA-EIA specification 568-A, covering for unshielded, screened and shielded twisted pair communication cables (UTP, ScTP and STP, respectively) These requirements have further limited the choice of the materials used in such cables, namely:
(a) the insulation materials for the single conductors, and
(b) the jacketing materials.
Given the stringent requirements of the UL 910/NFPA 262 test and the ANSI/TIA/EIA-568-A specification, few data communication cable constructions have qualified to date for installation in plenum spaces without the use of metal conduits, hence called plenum data grade cables.
Until recently, the most economical materials suitable for cables meeting the requirements of the ANSI/EIA/TIA specifications and qualifying for plenum cables consist of the following combination:
Insulation: Conductors insulated with Fluorinated Ethylene Propylene (FEP) copolymer.
Jacket: Flame-retardant and low-smoke polyvinyl chloride based polymer alloys. EthyleneChloroTriFluoroEthylene (ECTFE) copolymer was also used but has been less popular due to higher price and rigidity of the resulting cables.
The use of FEP is a major inconvenience due to its high relative cost and limited availability. In recent developments, the use of FEP was reduced by the introduction of polyolefin (PO) substitutes. Applicant""s U.S. patent application Ser. No. 08/527,531 and the Canadian patent application Serial No. 2,157,322 disclose a cable design that meets the UL-910/NFPA 262 qualification tests and the ANSI/TIA/EIA specification containing polyolefin substitutes.
Polyolefin substitutes for fluoropolymer insulation materials such as FEP include the following: the replacement of the insulation material of one or more, or all, of the conductors of a cable by a polyolefin (PO) material, or by a dual layer insulation construction where the first layer consists of a solid or cellular polyolefin material and the second layer is a fluoropolymer, or by a combination of the two alternatives. The polyolefin material could contain flame retardant additives, and/or could contain smoke suppressant additives, where all additives may or may not contain halogens. MFA and PFA are fluoropolymers having equivalent physical and electrical properties as FEP, and which can be processed very similarly to FEP, but are relatively more costly. Therefore when FEP is mentioned, MFA and PFA are included within the discussion.
With polyolefin insulation substitutes, thicker jackets are required in order to meet the UL-910/NFPA 262 qualification tests resulting in higher costs for the jacket per unit length of cable, and more difficulties during installation due to its higher rigidity.
In addition, concerns were raised regarding the long term performance of cables jacketed with flame retardant and low smoke polyvinyl chloride based polymers when exposed to high humidity and temperatures. In particular, the exposure of such cables to 95% humidity and 95xc2x0 F. for as little as 300 hours was demonstrated to cause a significant increase in the signal attenuation of the cable.
Another important design requirement in data transmission, is the overall shielding of cables (SCTP or S-UTP) in order to avoid electromagnetic energy being radiated from the cable and/or to the cable. This is especially true for structured cabling systems requiring transmission frequencies of up to and around 100 Mhz and higher. The known art consists of applying a metal foil tape, or a metal coated polymer tape, with or without a wire braid around the cable core of insulated conductors prior to the application of the jacket. A grounding conductor in contact with the metallic foil is also applied. The metal foil tape or metallic coated polymer tape (shielding tapes or metallic foil tapes), with or without the wire braid, when properly applied and electrically grounded, will shield or screen away the electromagnetic energy being emitted from a cable into the external environment or protect a cable from interference by external sources.
The proximity of a metallic foil shield and/or a wire braid shield around the insulated conductors requires a substantial increase in insulation thickness, in order to meet signal attenuation results and a characteristic impedance equivalent to that of an unshielded cable.
The application of an efficient shield with 100% coverage consisting of a metallic foil tape with closed overlapping edges all along the length of the cable is a difficult task, due to the irregular shape and instability of the cable core. Opening of the tape overlap may occur and cause leakage or penetration of electromagnetic energy when the cable is in use.
The installation of shielded cables requires the additional manipulation of the shielding tape, the wire braid (if any), and the grounding wire during the connectorization with high density cross-connect devices or during the installation of shielded connectors.
A need therefore exist for an electrical cable which overcomes the problems of the aforementioned prior art cables.
Accordingly, it is an object of the present invention to provide an electrical cable which reduces the need for FEP or other costly fluoropolymer alternative insulation materials for plenum UL-910/NFPA 262 test qualifications of UTP, ScTP and STP data grade cables, while providing high speed data transmission performance and which can simultaneously achieve a lower jacket thickness and a lower overall cost per unit length.
Yet another object of the present invention to provide an electrical cable which improves the shielding effectiveness of plenum and non-plenum rated cables from emitting or receiving electromagnetic energy, by using a metallic shield in the form of tape or metallic coated polymer tape with or without a wire braid.
Yet another object of the present invention is to provide a dual layer screened (ScTP) cable wherein the addition of another layer between the conductors and the shielding provides the cable with electrical signal attenuation and impedance characteristics equivalent to that of an unshielded cable with similar conductor insulation thicknesses.
Yet another object of the present invention is to provide a screened (SCTP) cable with a conductor insulation thickness which is similar to or greater than that of an unscreened (UTP) cable but less than the conductor insulation thickness of prior art screened cables.
Yet another object of the present invention to provide an electrical cable which lowers the cost of manufacturing and installation of SCTP and STP cables for plenum and non-plenum applications
Yet another object of the present invention to provide an electrical cable which increases the dielectric strength between the conductors and the shield.
Yet another object of the present invention to provide an electrical cable which protects the cables from possible transmission performance deterioration due to exposure to high temperature and relative humidity.
According to an aspect of the present invention, there is provided a shielded electrical cable having at least a pair of insulated conductors, a metallic shield and a jacket surrounding the shield and insulated conductors, comprising:
a first jacket layer surrounding said insulated conductors;
a metallic shield surrounding said first jacket layer; and
a second jacket layer surrounding and sealing said metallic shield against said first jacket layer, said second jacket layer being made of flame retardant material.
According to another aspect of the present invention, there is provided a plenum rated electrical cable having at least a pair of insulated conductors and a jacket surrounding the insulated conductors, comprising:
a first jacket layer surrounding said insulated conductors, said first jacket layer being made of a low-smoke and flame-retardant material; and
a second jacket layer surrounding said first jacket layer, said second jacket layer being made of flame retardant and low smoke material.